Incorporating canopy physiology into a hydrological model: Photosynthesis, dynamic respiration, and stomatal sensitivity

Vegetation modulates the effects of climate variability through soil-vegetation-atmosphere (SVAT) interactions. A quantitative understanding of such interactions requires the proper integration of the water cycle and photosynthesis. While biochemical models have widely been used to estimate primary production, the effects of water stress on transpiration and carbon assimilation rates, and its feedbacks into the water cycle are not generally represented. The objective of this study is to investigate the limiting effects of soil moisture and evaporative demand on photosynthesis, and to understand its interactions with other hydrological processes. Our approach consists of integrating a physically based land surface hydrological model (LSHM) with a biochemical model for leaf photosynthesis and a substrate-structure separation model for respiration, including parameterizations of the diurnal cycle of Rubisco concentration and species-specific stomatal conductance (resistance). Exploratory simulations to evaluate the model against results from previous studies indicated that the model captures basic processes of canopy physiology well. Sensitivity analysis shows that water stress at sub-daily time-scales is an important limiting factor of photosynthesis, thus constraining carbon assimilation. On the other hand, the results further suggest that the biological control of transpiration via stomatal sensitivity is only significant under soil water stress conditions. Overall, the integrated model is capable of estimating not only carbon assimilation, but also the length of the growing season, as well as feedbacks between vegetation and soil hydrology processes.